The present invention relates generally to continuous methods of making beaded retroreflective products. The invention has particular application to methods of making bulk quantities of retroreflective end-use materials such as fabrics for use or application in shoes, vests, backpacks, handbags, appliques, or similar personal belongings.
The reader is directed to the glossary at the end of the specification for guidance on the meaning of certain terms used herein.
The use of beaded retroreflective fabrics (sometimes referred to in the literature as reflective fabrics) to increase the visibility of pedestrians has long been known. Such fabrics have the property of reflecting incident light, such as light from a vehicle headlamp, back in the general direction from which the light originated, regardless of the angle at which the incident light impinges on the surface of the fabric. Thus, a person wearing such a fabric can be highly visible to drivers of such vehicles at night, depending on (i) the amount of retroreflective fabric used, and (ii) the reflectivity of the particular fabric.
The retroreflectivity is provided by a layer of tiny glass beads or microspheres that cooperate with a reflective agent such as a layer of aluminum. The beads are partially embedded in a binder layer that holds the beads to the fabric, and partially exposed to the atmosphere. Incident light enters the exposed portion of a bead and is focused by the bead onto the reflective agent, which is disposed at the back of the bead embedded in the binder layer, whereupon the light is reflected back through the bead, exiting through the exposed portion in a direction opposite to the incident direction. This type of construction is referred to as an xe2x80x9cexposed lensxe2x80x9d retroreflector, because it uses microspheres with portions that are exposed to the atmosphere.
There is a wide variety of such beaded retroreflective fabrics available today from a number of manufacturers. However, the methods currently used to make such fabrics on a commercial basis fall into three basic types.
The first type of known process is referred to herein as the xe2x80x9crandomized beadxe2x80x9d process. In it, a solution that consists of a water-based ink, specially prepared beads, and a coupling agent is applied to the fabric by screen printing, or alternatively by flexographic or continuous roll printing. Each specially prepared bead has been provided with a hemispherical coating of aluminum. When the solution dries, at least some of the beads protrude from the bead bond that secures them to the fabric. No attempt is made to orient each bead so that the uncoated portion is exposed to the air and the aluminum-coated portion is embedded in the bead bond, but in practice enough beads are so oriented (because of their random alignment) so that the treated fabric achieves a reflectivity of up to about 60-70 cd/(luxxc2x7m2), and more typically about 25-30 cd/(luxxc2x7m2). An advantage of this process is its simplicity, but a major drawback is the relatively low reflectivity achieved.
The second type of process is referred to as the xe2x80x9cbead dropxe2x80x9d process. In it, a resin that contains tiny flakes of aluminum is roll coated onto the fabric. The coated fabric is then passed through a bead application station, where uncoated glass beads are dropped onto the resin from a bead reservoir above the web. The beads sink partially into the resin. Finally, the web is passed through an oven that cures the resin. Because of the optically inefficient distribution of the aluminum reflector, fabrics prepared using this process achieve reflectivities of only about 30-90 cd/(lux xc2x7m2) at best. Like the randomized bead process, the bead drop process is relatively simple to implement in production, but it also achieves only low reflectivity values.
The third type of known process, referred to as the xe2x80x9crelease linerxe2x80x9d process, is more complex and generally more expensive than the other two, but can produce high performance retroreflective fabrics having reflectivities of 500 cd/(luxxc2x7m2) or more. FIGS. 1-3 depict some aspects of a representative process, variations of which can be found in published literature, e.g. U.S. Pat. No. 3,172,942 (Berg), U.S. Pat. No. 5,344,705 (Olsen), U.S. Pat. No. 5,474,827 (Crandall et al.), and U.S. Pat. No. 5,510,178 (Olsen et al.). FIG. 4 depicts a portion of a reflectorized fabric made by such representative process. In FIG. 1, a carrier layer 10 comprising a paper sheet 12 and a heat-softened polymer lining 14 (see detail in FIG. 1a) passes underneath a bead application station 16. Glass microspheres or beads 18 cascade from a reservoir 20 down onto the carrier layer. The beads 18 sink partially into the lining 14, forming a monolayer of beads, portions of which are exposed (see detail in FIG. 1b). After the lining 14 has cooled, the carrier layer 10 with the monolayer of beads 18 is wound up into a roll so that it can be transported to a vacuum chamber 21, shown in FIG. 2. The carrier layer roll is unwound in the vacuum chamber 21 so that, at a metal coating station 22, a specularly reflective metal 24 such as aluminum can be applied to exposed portions of beads 18 and to any exposed portions of lining 14, forming a reflective aluminum film 26 (see detail in FIG. 2a). The carrier layer is wound up, removed from the vacuum chamber, and unwound again for the next operation, shown in FIG. 3. In that figure, a layer of prebinder composition 28 is applied by roll-coating to the aluminum-coated monolayer of beads. This yields a carrier layer web the details of which are substantially as shown in FIG. 3a. The fabric 30 to be reflectorized is then introduced and brought into contact with the prebinder composition 28. In FIG. 3, this is shown by passing the fabric 30 and the carrier layer with its various coatings through a nip formed between rollers 32. FIG. 3b shows the details of the resulting web. The web so constructed then passes through an oven 31, where prebinder composition 28 solidifies. In a final step (not shown), the carrier layer 10 is stripped away and discarded to make fabric 30 retroreflective as shown in FIG. 4, by virtue of the partially exposed monolayer of beads 18 cooperating with the underlying reflective film 26, both held to the fabric by binder layer 28. Such a fabric can achieve reflectivities of 500 cd/(luxxc2x7m2) or more because of the consistent placement of the reflective aluminum film on the embedded portion of each bead.
Each of the three above-described processes has been used commercially since at least about the mid-1970s. However, until now there has been no continuous process that is both: (i) considerably simpler than the release liner method, and (ii) capable of producing highly retroreflective fabrics. This is so despite a general awareness of other methods of making certain exposed lens retroreflective articles, including the method of U.S. Pat. No. 3,790,431 (Tung), where a binder material is coated on an open web type fabric, microspheres completely covered with a reflective material are applied to the coated fabric, the binder material is dried or cured, and reflective material covering exposed surfaces of the microspheres is removed, as by etching. See also U.S. Pat. No. 3,934,065 (Tung); U.S. Pat. No. 3,989,775 (Jack et al.); U.S. Pat. No. 4,005,538 (Tung); and 4,678,695 (Tung et al.). Thus, there has been a long-felt need for a relatively simple continuous process capable of making large quantities of high performance reflectorized fabric.
Disclosed herein are continuous processes capable of manufacturing commercial quantities of high performance reflective fabric, i.e., fabric having a reflectivity of at least about 100 cd/(lux xc2x7m2), but which are greatly simplified compared to the release liner method because they do not require the use of any release liner.
Instead, in the disclosed processes, an extended length of fabric is provided such as by unwinding an input roll of the fabric to be reflectorized, and the fabric is then passed through a coating station that applies a coating of binder material to the fabric. The fabric then passes through a bead application station, where aluminum-coated beads are applied to the coating of binder material. An etching station receives the fabric and removes exposed portions of the aluminum coating from the beads. The disclosed processes thus convert standard fabric to reflectorized fabric in a continuous fashion suitable for handling bulk quantities, and are easily capable of achieving reflectivities of well over 100 cd/(luxxc2x7m2).
In some disclosed processes, beads are distributed over substantially the entire front surface of the fabric, while in others the beads are provided on only parts of the front surface, such as in the form of indicia. Techniques are disclosed for achieving a high packing density of beads, which has been found elusive for fully aluminum-coated microspheres. The various steps of the process can be carried out in an uninterrupted sequence, or if desired the fabric can be wound up, stored or transported, and unwound between some of the steps. Advantageously, process steps carried out in a vacuum chamber and the associated expense and added difficulty can be avoided since a reflective vapor coat need not be applied to the web itself.